1. Field of the Invention
The invention is directed to methods and compositions for improving the adhesion properties and switching performance of electrophoretic displays, particularly at low operation voltages.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
The latest improvement to EPD technology involves the use of microcup technology. The microcup technology is a significant advance in EPD display technology, allowing for roll to roll manufacturing.
Although EPD technology has advanced, its performance, however, can be further improved. In a co-pending application, U.S. Ser. No. 10/618,257 filed on Jul. 10, 2003, the content of which is incorporated herein by reference in its entirety, methods are disclosed for improving display performance. However, when a high absorbance dye or pigment is incorporated into the adhesive layer (15) and carbon black in the sealing layer (14), the display is limited to be viewed from the side of the second electrode (12 as shown in FIG. 1) because the opaque color of the sealing and adhesive layers from the dye and carbon black makes viewing from the side of the first electrode (11 as shown in FIG. 1) difficult.
Another area for improvement in known EPD devices lie in the adhesives currently used. Typical examples of the lamination adhesives include acrylic and rubber types of pressure sensitive adhesives (such as DURO-TAK 80-1105), hot melt adhesives (such as EVA, polyester or polyamide types), UV adhesives (based on multifunctional acrylates, vinylethers or epoxides) and thermal or moisture cured adhesives (such as epoxy resins, polyurethanes, vinyl esters or rubbers). Most of these conventional adhesives exhibit a strong capacitor effect that often causes inferior EPD switching performance. The use of a hydrophilic adhesive or addition of a conductive additive in the adhesive may alleviate the problems associated with the strong capacitor effect. However, these possible remedies may result in setbacks such as sensitivity to humidity and undesirable current leakage or short circuitry.